1. Field of the Invention
This invention relates to an electro-luminescence display (ELD), and more particularly to an active matrix ELD and a method of driving the same.
2. Description of the Related Art
The ELD is a display device in which electrons and holes are injected from the exterior thereof to re-combine the electrons with the holes and thus produce excited molecules so as to exploit the luminescence of these excited molecules. ELD devices are gaining popularity, due to their thin display panel size and relatively low power consumption, because ELDs do not require a backlight device.
FIG. 1 is an equivalent circuit diagram of a unit cell in the conventional ELD. In FIG. 1, a plurality of gate lines G cross a plurality of data lines D to define pixel cell areas therebetween. In the pixel cell area, a power supply line L is arranged in parallel to the data line D. Alternatively, the power supply line L may be arranged in parallel to the gate line G. The pixel cell area includes a switching device T1, a driving device T2, a storage capacitor C and an electro-luminescent (EL) diode EL. The switching device T1 has a gate connected to the gate line G, a source connected to the data line and a drain connected to a gate of the driving device T2. The drain of the driving device T2 is connected to an anode (+) of the EL diode EL while the source thereof is connected to the power supply line L. A storage capacitor C is connected between the gate of the driving device T2 and the power supply line L. A cathode (−) of the EL diode EL is connected to a common electrode terminal 10.
In the ELD having the structure as described above, if the gate line G connected to the switching device T1 is selected by a gate driver (not shown) to be turned on, then a data signal from the data line D connected to the switching device T1 is stored in the storage capacitor C. When the switching device T1 is turned off, a voltage of the storage capacitor C is maintained until the gate line G is selected again. At this time, the storage capacitor C has a voltage applied between the gate and the source of the driving device T2. Thus, a source current determined in accordance with a gate voltage of the driving device T2 arrives at the common electrode 10, via the driving device T2 and the EL diode EL, from the power supply line L. In this operational process, the EL diode EL becomes luminous. In this manner, the driving device T2 responds to a selecting signal applied to the gate line G, and to a data signal applied to the data line D to control a current flowing through the driving device T2 from the power supply line L. The EL diode EL is luminous at a desired magnitude of brightness corresponding to the magnitude of current applied by the driving transistor T2. For example, if a certain gate voltage is applied to the gate of the driving device T2, then the magnitude of a current passing through the driving device T2 is determined. Accordingly, the magnitude of a current flowing through the EL diode EL also is determined.
FIG. 2 is a view showing the structure of a conventional ELD, which illustrates a substrate on which a pixel cell emitting a red light (R), a pixel cell emitting a green light (G) and a pixel cell emitting a blue light (B) are arranged. Since the basic structure of each pixel cell, with the exception of the driving devices, is identical to those described above with reference to FIG. 1, an explanation of the same elements will be omitted for the sake of brevity. In FIG. 2, a number of gate lines G1, G2, etc. cross a number of data lines D1, D2, D3, etc. to define a number of pixel cell areas therebetween. Each pixel cell area includes power supply lines L1, L2, L3, etc. The power supply lines L1, L2, L3, etc. provided in each pixel cell area are commonly connected to a single wiring 20 to commonly receive a voltage from a supply voltage terminal 21. Each pixel cell area is provided with a switching device T1, a driving device T2, a storage capacitor C and an EL diode EL. A common electrode terminal 22 plays a role to connect the EL diodes EL with one another. Each pixel cell can be defined as a “R” pixel cell emitting red light, a “G” pixel cell emitting green light and a “B” pixel cell emitting blue light, depending on the luminous color which each EL material constructing the EL diode EL emits. The R, G and B pixel cells are arranged in such a manner that three pixel cells make a group.
The group comprising an R pixel cell, G pixel cell and B pixel cell determines and displays a single color y, which is the combination of three colors. The display has a different color design in accordance with how to revive a color in accordance with various environmental conditions. A realization of the selected white color according to how to combine basic color (i.e., R, G and B), lights, which is hereinafter referred to as “white balance”, is determined by the chromaticity and brightness of the basic colors.
As shown in FIG. 3, however, each EL diode EL of the R, G and B pixel cells has a different brightness characteristic according to the applied current. In other words, when a current with the same magnitude flows in each pixel cell, the EL diode R-EL of the R pixel cell, the EL diode G-EL of the G pixel cell and the EL diode B-EL of the B pixel cell have a brightness magnitude different from one another. In the ELD, the brightness of red, blue and green lights required to meet the white balance is different from one another. This is because the EL materials making up the EL diode EL in each pixel cell are different.
Therefore, the conventional ELD fails to provide proper light realization even when identical driving waveforms are applied to each pixel cell. In other words, since the brightness according to a current flowing in each EL material composing the EL diode is different, a brightness required for each pixel cell can not be achieved like in a liquid crystal display (LCD) employing a color filter when applying identical data driving waveforms to the R, G and B pixel cells.
Accordingly, it is necessary to configure such a data driver that can independently drive the R, G and B cells for the sake of driving the ELD. As a result, the conventional ELD has the problems of a complicated design and a high production cost.